Non-circular screws



Nov. 25, E969 sr-:Ui oMoTo 3,479,923

NON-CIRCULAR SCREWS Filed July 17', 1967 2 Sheets-Sheet l Nov. 25,1969 sraul oMoTo NON-CIHCULAR SCREWS 2 Sheets-Sheet 2 Filed July 17, 1967 United States Patent O 3,479,921 NON-CIRCULAR SCRIEWS Seji Omoto, Tokyo, Japan, assignor of one-half to Hideo Osawa, Toyokawa-machi, Toyokawa-shi, Aichi-ken,

`Iapan Filed July 17, 1967, Ser. No. 653,713 Int. Cl. F16b 33/02 U5. Cl. 85-46 1 Claim ABSTRACT OF THE DISCLOSURE Fluteless swaging taps and self-tapping screws in which radial reliefs are disposed between adjacent thread forming areas but the roots of the threads are maintained at a constant distance from the axis of the screw, the tops of the threads dening a surface whose cross-section is noncircular. The forms of the screw threads are especially t for being produced by a rolling operation.

BACKGROUND OF THE INVENTION This invention relates to screws, and more particularly to non-circular screws of the self-tapping or thread forming type.

Non-circular screws have hitherto been proposed in which not only the crest portions of thread are at gradually varied radii but its root portions are also at correspondingly varied radii. In some prior screws, the crest portions of thread have been blunted or attened at spaced circumferential intervals about the thread to provide for radial reliefs. These screws may well be adapated for being produced by a shaving or grinding operation, but are rather troublesome to make by a rolling process. They are not strong enough to be driven forcibly into the stock.

The present invention aims to eliminate these drawbacks.

SUMMARY OF THE INVENTION The present invention resides in the provision of noncircular screws in which the thread is continuous but is gradually reduced in height at suitably spaced intervals along the thread to provide for reliefs while the root of the thread is kept at a constant radius throughout. The reduction in height of the thread is effected in such a manner that (l) the crest of the thread is gradually lowered with the crest angle remaining the same as the original one, (2) the crest of the thread is gradually lowered by a constant crest angle which is greater than the original one but is the same as the crest angle at the lowest thread, or (3) the crest of thread is gradually lowered with gradually increasing angle till the lowest thread is reached where the crest angle is the largest.

In order that the invention described herein may be r more fully understood, the meaning of certain terminology as used in the specifications will be set forth. The term non-circular refers to a form of a screw in which the surafce defined by the tops of the threads is of a noncircular conguration. In contrast, the term circular screw refers to an ordinary screw which has a ridge of uniform section in the form of a helix on the external surface of a cylinder. The term without lead, as used hereinafter in this specication, refers to an imaginary thread which would have no lead or in other word which would not advance axially Iwith respect to a fixed mating thread in one revolution.

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Generally speaking, non-cricular screws may be grouped into three types, namely (I) a screw in which the crest of the thread and also the root of the thread are both varied in height; that is the cross-sectional shape both at the crest of the thread (without lead) and at the root are such that any radius is not of the same length, (II) a screw in which the cross-sectional shape at the crest of the thread is such that any radius is of the same length while at the root of the thread any radius is not of the same length, and (III) a screw in Vwhich the crosssectional shape at the crest of the thread is such that any radius is not of the same length while at the root of the thread any radius is of the same length.

Non-circular screws which have hitherto been proposed belong to said type (I). The present invention is primarily concerned with non-circular screws of type (III), while at the same time the invention is intended to include noncircular screws of type (II) which are exactly the reverse con-formation of type (III).

It is a primary object of the invention to provide a novel and improved screw of a non-circular configuration particularly adapted for production by a rolling operation.

It is a further object of the invention to provide a fluteless swaging tap which requires low torque for driving to form mating threads on the stock.

It is another object of the invention to provide a selftapping screw which has a strong force for clamping and also a high tensile strength.

It is still another object of the invention to provide a newly shaped non-circular screw whose thread is easily and eciently generated by the use of a ator cylindrical-die method of rolling process.

These and other objects and advantages of the invention will be more clearly understood and appreciated from the following detailed description read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIGURE l is a perspective view of a cylinder shown as a base surface from which the thread according to this invention is to project.

FIGURE 2 is a diagrammatic perspective view, on an enlarged scale, of a fraction of thread, straightened and developed on a plane, showing the thread formation of one embodiment of this invention.

FIGURES 3 and 4 are similar to FIGURE 2 but showing second and third embodiments of this invention.

FIGURES 5, 6, and 7 are end views .of the threads illustrated in FIGURES 2, 3 and 4, respectively.

FIGURE 8 is a transverse sectional view of a screw, on a reduced scale, through the root of thread showing the thread outline of both the first and second embodiments but without lead.

FIGURES 9 and 10 are similar to FIGURE 8 but showing the third and fourth embodiments, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS For explanation, reference is made to FIGURE 1, in which the numeral 10 designates a right circular cylinder having a longitudinal axis at O O. It is obvious that any point on its surface is equi-distant from said axis. Now if one or more threads of uniform section are Wound on the external surface of said base cylinder consecutively and on a constant pitch in the form of helix, then a normal screw will be obtained. According to the present invention, the thread is not of uniform section but is gradually varied in height and form in such fashion as will fully be explained in connection with FIGURES 2 to 7.

Generally speaking, applicants screw is of non-circular configuration of the type in which every point of the crest is varied in height while the root points are all at the same radii. Referring now to FIGURE 2, a fraction of thread 21 according to one embodiment of the invention is diagrammatically shown as developed on a plane. A triangular shape shown at the left end is a lateral section of the thread at its highest portion 22. Straight line designates a base line which, in the case of a screw without lead, coincides with a generating line of the surface of cylinder 10 shown in FIGURE l. As is clear from FIGURE 2, the crest height of the thread gradually decreases from the highest thread portion 22 to the lowest thread portion 23 and then increases from that portion to the next highest portion 22 and so on, longitudinally of the thread. The manner in which the crest height is changed is illustrated more clearly in FIGURE 5. The crest angle of the perfect thread or the highest thread portion 22 is kept constant throughout the change of height until the lowest thread portion 23 is attained. The crest angle along crest line 24 is constant and is nowhere flattened theoretically. Reference is made to theoretically because no crest has a sharp apex in actual practice. On the other hand, the root of the thread has a fiat part 25 in the region of lowered thread portion, said fiat part 25 resembling something like a half of a leaf as in FIGURE 2. Actually this part 25 is not fiat but is a part of a cylindrical surface.

Referring to FIGURE 3, the numeral 31 indicates a modified form of thread, in which crest line 34 is gradually varied in height in the manner similar to crest line 24 of FIGURE 2, but the crest angle is also varied increasingly from the highest thread portion 32 to the lowest thread portion 33 as is more clearly illustrated in FIGURE 6. In this form of thread, neither crest nor root has no flat part theoretically Referring to FIGURE 4, the numeral 41 designates another modification of thread, in which crest line 44 is gradually varied in height in the manner similar to FIGURES 2 and 3, but the crest angle, except at the highest thread portion 42, is made throughout the same as that of the lowest thread portion 43 which is greater than the former, as is more clearly illustrated in FIGURE 7. Thus, a lateral section at any point longitudinally along the thread is of the shape of gambrel roof with the exception of both at the highest portion 42 and at the lowest portion 43 where it is a triangle. This form of thread may be made in practice by a tool or grinding wheel formed with a V-shaped cutting edge. For example, a thread of uniform section having 60 degree crest angle may be changed into the form of FIGURE 4 by shaving off the top gradually from the perfect thread portion or the highest portion 42 toward the lowest portion 43 with the use of a V-shaped tool of 90 degree cutting edge. It will be apparent that neither crest nor root of thread in this embodiment has any flat part theoretically, but a second ridge line 44 is formed on the ank on either side of crest line 44. Said ridge line 44' is the line of junction between the flank of the original thread and the shaved surface of the finished thread.

In the forms of thread illustrated in FIGURES 2 to 4, the lowest thread portions 23, 33 and 34 are disposed substantially at mid-point between two adjacent perfect thread portions 22, 32 and 42 of threads 21, 31 and 41, respectively. The fashion in which the crest height changes along with its angle is shown in FIGURES 5, 6 and 7 which correspond to FIGURES 2, 3 and 4, respectively.

As previously stated, the form of thread 21 illustrated in FIGURE 2 has fiat part 25 at the root of thread. When this form is utilized as a tap for swaging a mating female screw or as a bolt for self tapping, there occurs no interference of thread in said flat part 25 and no disturbance is observed to lessen the efficiency of the thread. However, when this type of screw is produced by a rolling process, sometimes fine-grained material is observed to peel off in this fiat area though it does not seriously affect the swagng action of thread. Therefore, it is preferred in practice to utilize such patterns as threads 31 and 41 illustrated in FIGURES 3 and 4.

For the sake of convenience, it has so far been stated that a non-circular screw according to this invention has such configuration as may be obtained by closely winding a thread or threads of varying crest height (21, 31 or 41) in the form of helix around a cylinder 10 of the same radius. It should, however, be understood that said cylinder 10 can easily be substituted by a cone or frustum of a cone and the same undulating thread or threads may be closely wound thereon substantially in the same manner as above stated. The resulting type of screw will be a tapered one of a non-circular configuration. In practice this type will be formed on a tapered portion of a screw, an entering end of a tap, or a tapered portion of a back-taper screw. This type of screw configuration having a conical base may be made without departing from the inventions fundamental theme.

As is clear from the explanation so far stated, the screw according to this invention is characterized in that the sectional shape thereof through a plane perpendicular to the axis is a circle at the root of thread (without lead), the crest of thread has theoretically no flat part, and the crest line of the thread is of a wavy formation. It is. therefore, fundamentally different from such forms of screw as have hitherto been proposed, for example, in connection with a fluteless swaging tap in which the crest angle of the thread is constant throughout while the crest and root are both varied in height circumferentially to provide radial reliefs so that cross-sectional shapes of the tap (without lead) at crest and root may both be of a noncircular configuration.

In contrast, applicants screw has no recess at the root of the thread, and accordingly it is stronger than any other non-circular screws of prior art. Particularly, in the forms of thread in which the crest angle is increased according as the crest height is reduced, the strength of the thread per se can also be increased.

Applicants screws, however, are very difficult to make by an ordinary grinding or cutting process, with the exception of a certain form. On the contrary, it is easy to make them by a rolling operation with the use of dies which have working faces provided with such unevenness as exact reversal of that of non-circular screws to be produced. It is true that applicants screws are most suited for production by a rolling process for the following reasons: As the forms of thread according to this invention have no recess in the root, it will be understood that the dies for making said screw will have Working faces formed with reversed unevenness and therefore the tops of ridges of the Working faces of the dies, which correspond to the roots of thread of the desired screw, will all be arranged on a plane surface, when fiat dies are used. and on a cylindrical surface, when cylindrical dies are used. Consequently, when such dies are used, regardless of whether they are fiat or cylindrical, in the rolling process, the operation can start easily without jolting, and initial deformation plastically of the stock takes place very smoothly and then subsequent deformation will also proceed equally Well until the finished product is obtained. Easiness and efiiciency of this operation are almost the same as when ordinary circular screws are being produced by die rolling.

In the case of conventional non-circular screws which have height Varying crests and depth varying roots, the Working faces of the dies therefor are not flat or cylindrical but are irregularly ridged due to the uneveness at the roots of the screws. When the tops of ridges have been arranged on lines parallel to the longitudinal axis of the screw to be produced, the rolling operation therefor has been extremely diicult, with one exception of the form which is generally referred to as equidiametral but non-circular shape (i.e. in the cross-sectional shape of this form of screw, the outline is non-circular but each high point is directly opposite a low point so that any diameter is of the same length). In order to eliminate the above-mentioned diiculty, it has been necessary to arrange the tops of ridges on lines not parallel but obliquely to the axis so as to insure smooth rolling. In contrast, the dies for applicants screws have working faces either at or cylindrical, so there is no diiculty in rolling such screws, even if the tops of the threads are arranged on lines parallel to the axis, as previously mentioned.

The number of highest crest or perfect thread portions 22, 32 or 42 must be more than one per one revolution of threads 21, 31 or 41 (FIGURE 2, 3 or 4) around the base cylinder (FIGURE 1) in the form of a helix. It is preferable to provide, per one revolution of the thread, two to six highest portions (22, 32 or 42) appropriately spaced from one another circumferentially as seen in FIGURES 8 to l0. The arrangement of said highest portions longitudinally of the screw may be such that they are disposed on lines either parallel or obliquely to the axis of the screw. If the lines are oblique, then they will become helical lines which, however, are quite different from those of thread formation and may be referred to as helical lines of highest portions. In order to facilitate the rolling operation, it is preferable to arrange the highest portions on such helical lines which may be either right-handed or left-handed, though it is possible in the invention to arrange them on parallel lines to the axis of the screw as previously stated. However, the helical arrangement is preferred to the parallel one because the working faces of dies for the former are easy to make, and furthermore mating screw threads rolled by such dies are better than the latter.

As already set forth, non-circular screws according to the invention comprise such threads as are shown in FIG- URES 2 to 7, whose crests are gradually reduced in height from perfect thread portions 22, 32 or 42 to lowest portions 23, 33 or 43 in such a manner that the lateral sectional shape at any point along the thread is (l) of analogous form, (2) of gambrel roof form, or (3) of increasing crest angle. In the manufacture of dies for rolling such non-circular screws, however, the work in die making is facilitated if the above three forms of thread are properly mixed into one variety. Therefore such a mixed form of thread is generally used in practice rather than utilizing the three forms separately.

Now referring to FIGURES 8 to l0, lateral sections at the root of thread (Without lead) are shown as having the thread outline of a non-circular configuration according to the invention. By way of example, each of the embodiments is shown as having six highest crest portions per one revolution of thread. In each section the root outline is shown as a circle 11.

FIGURE 8 shows a non-circular screw which has a thread of the form illustrated either in FIGURE 2 or in FIGURE 3. The outline of the lowered crest portions of the thread is shown approximately as a straight line. This line may be changed in any manner unless it departs from the idea of the invention. It is, however, preferable to make it in the form of a straight line or a line as nearly as straight but slightly curved inwardly or outwardly. This form of screw can be obtained by a rolling process with the use of such dies as prepared by the methods disclosed in the specifications of Japanese Patent Nos. 440,387 and 172,758, said dies being provided with exactly reversed unevenness to that of the non-circular screw which is desired to produce.

FIGURE 9 shows a non-circular screw which has a thread of the form illustrated in FIGURE 4, the outline of the lowered crest portions of the thread being shown similarly as a straight line. Between the thread outline and the root circle 11, there appears a Second ridge line. Thus, the perfect thread portion 42 is within range A, portion 43 where the crest is lowered with a constant angle greater than that of perfect thread portion 42r is within range C, and portion in the form of mixture of the two is within range B where a second ridge line appears.

FIGURE 10 shows a non-circular screw of this invention, which is not produced by a rolling operation but worked out of an ordinary circular screw by a shaving or grinding operation. This form. has also six perfect thread portions 52 left unground and uniformly spaced circumferentially, the remaining thread portions having been ground olf by a grinder having a V edge of an angle greater than the crest angle of portion 52. The numeral 53 indicates the lowered thread portions which are formed by feeding the grinder along the root circle 11. If desired, the grinder may be moved along a straight line or even along an excentric curve. However, the thread outline of portions 53 must always be inside a circumscribed circle of perfect thread portion 52, and the lowest position of the tool path should be limited by the root circle 11. The form illustrated in FIGURE 10 cannot be manufactured elficiently when compared with those illustrated in FIG- URES 8 and 9, and therefore this form is mainly utilized to make only a master screw for manufacturing rolling dies.

Non-circular screws of the form as heretofore stated are used mainly in the threaded portions of taps and self-tapping screws. Screws having exactly reversed thread formation are used in the threaded portions of rolling dies.

As the non-circular screws of this invention are constructed as above set forth, they are stronger than prior art non-circular screws, without injuring the relieving function. When they are utilized as threaded portions of female thread forming tools, such for example as taps, self-tapping screws or the like, driving torque is minimized and thread forming by plastic deformation is enhanced. Further, as these non-circular screws have particularly suitable formations for being rolled, they can be effectively and advantageously used in mass production.

While the invention has been described in its presently preferred forms, it is to be realized that many changes to the instant embodiments may be made without departing from the scope and concept of the invention.

I claim:

1. In screws of such a form .as may be obtained when a continuous thread having a crest progressively varying in height is closely formed around the external surface of a cylinder at a constant pitch in the form of a helix; a non-circular screw characterized in that said continuous thread has, in one turn thereof, a plurality of highest portions where the pattern of the perfect thread formation and the desired size thereof are maintained while the portions other than said highest ones are all reduced gradually in height, and the crest of said continuous thread is practically sharp and has no flat part thereon, the prole, in planes perpendicular to the longitudinal axis of the screw without lead, being a circle at the roots of the thread, but nearly a polygon at the crests, said continuous thread has an undulating crest line and the lateral section between the highest and lowest points are a series of sections each with a different shape Whose sides have two slopes, the upper and lower ones corresponding to those of the lateral sections, respectively, of the triangular shapes at the lowest and highest points, and further characterized in that the highest crest portions 0f the successive turns of the thread are arranged continuously longitudinally on helical lines so that the circumscribing surface thereof may present an appearance of a regular polygonal column which is twisted around and extends through its longitudinal axis.

(References on following page) 7 8 References Cited FOREIGN PATENTS UNITED STATES PATENTS 210,236 7/ 1960 Austria. 1,428,792 9/1922 Lysiak 85 46 1,004,883 9/ 1965 Great Brltam. 1876,796 9/1932 Tfboievich 85-46 f EDWARD C. ALLEN, Primary Examiner 2,336,570 12/1943 Rabezzana 85-46 0 3,180,202 4/1965 Kahn 85-46 U.s. C1. X.R.

3,200,691 8/ 1965 Neuschotz 85--47 85-47 

